A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant processor 108. Besides receiving the processed audio information, the implant processor 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrode contacts 112 on its surface that provide selective stimulation of the cochlea 104.
It once was commonly the case that cochlear implant systems were unilateral systems with only one ear being implanted with an electrode array that delivers electrical stimulation signals to the implanted ear. More commonly today, cochlear implant systems often are bilateral with both ears receiving implanted electrode arrays that deliver stimulation signals to the implanted ears.
The human auditory processing system segregates specific sound objects from complex auditory scenes using several binaural cues such as interaural time and level differences (ITD/ILD) and monaural cues such as harmonicity or common onset. This process is known as auditory scene analysis (ASA) as described more fully in A. S. Bregman Auditory Scene Analysis: The Perceptual Organization of Sound, MIT Press, Cambridge, Mass. (1990), incorporated herein by reference. Hearing impaired patients have difficulties successfully performing such an auditory scene analysis even with a hearing prosthesis such as a cochlear implant. Because of such problems, cochlear implant users often struggle to listen to a single individual sound source within a mixture of multiple sound sources as in a noisy sound environment. In the case of understanding speech, this translates into reduced speech intelligibility. In the case of music, musical perception is degraded due to the inability to successfully isolate and follow individual instruments.
U.S. Patent Publication 20100135500 describes a binaural hearing system with microphones on either side of the patient's head based on comparing the relative signal-to-noise ratios from each microphone. But there is no suggestion as to analysis and processing of sound objects in the surrounding sound environment.
WO 2013/101088 by Mishra stated that in prior art systems the sensed ipsilateral and contralateral signals were “compared as a whole and select one of them for presentation to the patient based on the comparison.” Mishra proposed to compare the ipsilateral and contralateral signals on a channel-by-channel comparison, selectively amplifying the corresponding ipsilateral and contralateral signals (FIGS. 4 and 6) and then finally mixing the modified channel signals which was forwarded to the implanted cochlear implant. This approach does not consider components of an audio signal which may be correspond to the same sound object, e.g. the fundamental and first and/or second harmonic may be treated in different ways (different gains) and thus the sound object may be distorted.